HOW MANY PEOPLE ARE OUT THERE?

An article by Dr Markus Kirkilionis, Mathematics Institute and Centre for Complexity Science

According to the UN Population Division, on Monday October 31st 2011 the population of the Earth hit 7 billion. Yet, statistically, the figure is highly unreliable, not least beacuse the countries with the highest fertility rates don't have reliable census data. Dr Markus Kirkilionis, who studies complex systems with the help of mathematical modelling and simulation, takes a closer look at the business of gathering population data.

On Monday October 31st 2011, according to the UN Population Fund report, the world population exceeded, for the first time in history, the threshold of 7 billion individuals. If this estimate is reliable the baby causing the excess will likely have been born in Uganda, one of the countries having currently the highest world-wide fertility rates.

The estimate of a 7-billion-individuals-large human population in 2011 is of course statistically highly unreliable. The main reason for this is simply that the countries with currently highest fertility rates do not have very reliable census data. Therefore the global estimates differ, for example the United States Census Bureau has a slightly lower estimate of 6.972 billion living humans on planet Earth.

Population censuses have nevertheless a long tradition in most parts of the world. The name census itself stems from the times of the Roman Republic where knowledge about population numbers were used to determine regional contributions to the Roman army. Nowadays accurate knowledge about different statistical data of a population is – more peacefully – essential for world-wide policy making, for example to distribute resources where they are needed. It is typically not possible to make a full census of an entire population in a whole region, therefore sampling techniques are used.

Whatever the future will be, we will have to be inventive and use our biggest resource: our brains.

In the best case, so-called probability sampling methods are applied. If we sample from a relatively high percentage of a certain regional population, with equal probability of each individual to be selected for the census, then this partial census should yield relatively accurate estimates of essential human statistics, like age, profession, income, education, value system-distribution etc. Such knowledge is essential to estimate fertility and death rates in a population. The total population number is then the result of a so-called birth-death process, where individuals are constantly born, but also die with a certain probability in a given time interval. If we are not dealing with a closed system like the entire Earth, say country-wide population statistics, then in addition we need to take into account the effects of immigration and emigration. Human mobility, either voluntarily or involuntarily, can have drastic effects on population growth in a given region.

Unfortunately one can imagine that applying probability sampling methods to demography is a rarely reached ideal in most parts of the world. Usually there are some bias effects that give some individuals a much lower chance to get selected for the census. This can be just unattainability in the broader sense. In some regions many people are not taking part in any global communication, they are not part of a social system, they even in some cases do not have an official identity. It is clear that this kind of unattainability is a problem for a census, irrespective of the statistical methods being used. Similar problems arise in the distribution of micro-credits in underdeveloped countries, once a much hailed mechanism to stimulate economic growth in the poorest parts of the world.

Besides these problems of having reliable census data there are a number of facts in demography that cannot be doubted. Population growth is still accelerating, i.e., world-wide fertility rates are higher than death rates. Some estimates forecast that 2070 might be the year where the two rates might eventually be equal. Thus this would be the year when the human population is in equilibrium. The question then will still be: how many people are out there? Will we be able to feed this large population in a sustainable way? Other facts are also mind buckling: there will be very likely, in, say 10 years' time, some populations in underdeveloped countries that are mainly consisting of young people below the age of 20. What kind of effects will this have? In contrast, other populations in developed countries will likely have many more old people than young people. What will this mean? Whatever the future will be, we will have to be inventive and use our biggest resource: our brains.

Dr Markus Kirkilionis is an Associate Professor of the Mathematics Institute and a Senior Associate of the Complexity Science Doctoral Training Centre at the University of Warwick. His chief research interests are mathematical biology, dynamic network models, complex systems, numerical analysis, pattern formation, physiologically structured Population models and (monotone) dynamical systems.